Traditionally, X-ray diagnostic processes record x-ray image patterns on silver halide films. These systems direct an initially uniform pattern of impinging X-ray radiation through the object to be studied, intercept the modulated pattern of X-ray radiation with an X-ray radiation intensifying screen, record the intensified pattern on a silver halide film, and chemically transform the latent pattern into a permanent and visible image called a radiograph.
Radiographs have been produced by using layers of radiation sensitive materials to directly capture radiographic images as modulated patterns of electrical charges. Depending on the intensity of the incident X-ray radiation, electrical charges generated either electrically or optically by the X-ray radiation within a pixelized area are quantized using a regularly arrange array of discrete solid state radiation sensors.
Recently, there has been rapid development of large area, flat panel, digital X-ray imagers for digital radiology using active matrix technologies used in large are displays. An active matrix consists of a two-dimensional array (of which, each element is called a pixel) of thin film transistors (TFTs) made with a large area compatible semiconductor material including among others, amorphous silicon, polycrystalline silicon, sputtered metal oxides, and organics. There are two general approaches to making flat-panel x-ray detectors, direct or indirect. The direct method primarily uses an amorphous selenium photoconductor as the X-ray to electric charge converting layer coupled directly to the active matrix. In the indirect method, a phosphor screen or scintillator (e.g. CsI, GdOS etc) is used to convert X-rays to light photons which are then converted to electric charge using an additional pixel level light sensor fabricated with the TFT on the active matrix array.
In prior art imaging systems, the pixel level light sensor disclosed is a vertical photodiode or alternately, a lateral metal-semiconductor-metal (MSM) photoconductor as taught in U.S. Pat. No. 6,373,062 B1: Interdigital photodetector for indirect x-ray detection in a radiography imaging system.
The key challenges with fabricating a vertical photodiode are the modifications required to the thin film transistor fabrication process specifically, thick amorphous silicon layers, specialized p doped contact layer and a complex RIE sidewall etching process to prevent optical crosstalk. These challenges reduce the fabrication yield and drive up the cost of manufacture. The key challenges with fabricating a lateral MSM photoconductor include the high dark currents at higher electric fields and photoresponse non-uniformity due to a non-uniform electric field. Both issues degrade performance, which is the key reason why MSM devices are not used in industry today for large area digital X-ray imaging. Accordingly, a need exists for a photodetector for use in flat-panel radiographic detectors that can mitigate the process yield and cost issues with vertical photodiodes while overcoming the device performance limitations in the MSM structures.